The multicarrier transmission system such as W-CDMA can transmit data with high transmission efficiency, by means of multiplex transmission using a plurality of subcarriers (carrier waves). At this time, when the phases of the plurality of subcarriers coincide, PAPR (Peak-to-Average Power Ratio) becomes large, namely, peak transmission power becomes extremely large as compared with mean transmission power. When transmitting a signal having a large PAPR, in order to prevent nonlinear distortion of a transmission signal in signal amplification and power leakage to adjacent channels, a transmission power amplifier is required to have high linearity over a wide dynamic range.
However, in general, the linearity and the efficiency of the amplifier are contradictory characteristics, and when securing high linearity over a wide dynamic range, power efficiency decreases, and power consumption in a communication apparatus increases. For this reason, to suppress PAPR, peak suppression processing for suppressing peak transmission power has hitherto been executed.
FIG. 1 is a diagram illustrating an exemplary configuration of a conventional radio transmission apparatus including a peak suppression unit. In the configuration shown in FIG. 1, peak suppression is executed in two stages. Specifically, an input transmission signal is input, via a delay unit 10, to a suppression unit 12 where first-stage peak suppression processing is executed. Also, a unit of the transmission signal is made to branch before being input to delay unit 10, and a suppression coefficient required for the first-stage peak suppression processing in suppression unit 12 is obtained. Specifically, a replica generation unit 14 generates a replica signal similar to a modulated signal, by oversampling the transmission signal, frequency converting through a band-limiting filter, synthesizing a plurality of subcarriers, and so on.
A peak detection unit 16 compares an envelope (amplitude of an envelope) of an output signal (replica signal) from replica generation unit 14 with a predetermined threshold A on the basis of each section (1 symbol or a plurality of symbols), and when there is an envelope exceeding the threshold A, a coefficient calculation unit 18 obtains a suppression coefficient for suppressing the maximum peak thereof to limit to the threshold A. The suppression coefficient is obtained as threshold A/maximum peak Pmax, for example. When there is no envelope exceeding the threshold A, ‘1’ is output as suppression coefficient. Suppression unit 12 for executing the first-stage peak suppression processing multiplies the suppression coefficient obtained in coefficient calculation unit 18 by the transmission signal, so as to suppress the peak of the transmission signal.
FIG. 2 is a diagram illustrating each transmission signal on which peak suppression is made. FIG. 2(a) is an example of a transmission signal in a predetermined section, of which peaks are suppressed in the former stage, while FIG. 2(b) is an example of a transmission signal in a predetermined section, of which peaks are suppressed in the latter stage. As shown in FIG. 2(a), among a plurality of envelopes Env0-Env3, the first-stage peak suppression processing multiplies the transmission signal in the predetermined section by the suppression coefficient so that an envelope (Env3) having a maximum peak Pmax among the peaks (P0, P1, P3) of the envelopes (Env0, Env1, Env3) exceeding the threshold A does not exceed the threshold A. Thus, the amplitude levels of the entire transmission signals in the predetermined section, namely the entire envelopes Env0-Env3 in the predetermined section, are compressed based on the maximum peak (the peak value of Env3).
An output signal from the suppression unit 12 is input to a modulation signal generation unit 20, so that a modulated signal (QPSK signal) is generated. Via a delay unit 30, the modulated signal is input to a suppression unit 32 in which second-stage peak suppression is executed. A unit of the modulated signal is made to branch before being input to delay unit 30, and a suppression coefficient necessary for the second-stage peak suppression by suppression unit 32 is obtained. Specifically, a peak detection unit 34 compares the envelope of the modulated signal with a predetermined threshold B, and when there is any envelope exceeding the threshold B, a coefficient calculation unit 36 obtains a suppression coefficient for suppressing the peak thereof to limit to the threshold B. Here, the threshold B is lower than the threshold A. Also, in order to suppress only an envelope(s) exceeding the threshold B in the second-stage peak suppression, the suppression coefficient is obtained on an envelope-by-envelope basis. The suppression coefficient is obtained as, for example, threshold B/peak Pi (i=0, 1, 2, . . . where each peak of Env0-Env3 after the first-stage peak suppression is defined as peak P0-P3), on an envelope-by-envelope basis. As to the envelope not exceeding the threshold B, ‘1’ is output as suppression coefficient. Suppression unit 32 executing the second-stage peak suppression processing multiplies the suppression coefficient obtained in coefficient calculation unit 36 by each envelope of the transmission signal, so as to suppress the peak of the transmission signal.
As shown in FIG. 2(b), by the second-stage peak suppression processing, the peak suppression is made for only the envelope exceeding the threshold B so that the peak thereof does not exceed the threshold B. For the envelopes Env0, Env1, Env3 exceeding the threshold B, the suppression coefficient is obtained for each envelope, and each peak P0, P1, P3 is suppressed to the threshold B.
An output signal from suppression unit 32 is input to a DA converter 40, and output from an antenna after amplification processing by means of a non-illustrated power amplifier, etc. are performed.
If the peak suppression processing is made before the modulation of the transmission signals, although the frequency spectrum of the transmission signal is not spread, there is a characteristic that EVM (Error Vector Magnitude) indicative of deviation from an ideal waveform in the IQ plane is deteriorated, while if the peak suppression processing is made after the modulation of the transmission signal, although the deterioration of EVM can be suppressed, there is a characteristic that the frequency spectrum is spread. Therefore, considering both characteristics, in order to optimize the distribution of the peak suppression, peak suppression processing is performed in two stages, as described above.